Various processes are known for fabricating electronic devices such as opto-electrical devices, photovoltaic devices, and liquid crystal display (LCD) devices. Commonly, these devices have been fabricated with a glass substrate and a conductor applied to the substrate which serves as an electrode. The conductor is first coated onto a side of the glass substrate, and then one or more additional layers are provided to complete the device. For example, in the case of an organic light emitting device (OLED), a transparent conductor such as indium tin oxide (ITO) may be coated onto a glass substrate to form an anode. Next, an electroluminescent layer comprising, for example, a blend of a hole transport polymer, an electron transport polymer and a light emissive polymer may be formed on the anode. Finally, a cathode is formed on the electroluminescent layer. The process of applying one or more of the layers may comprise steps that are carried out at an elevated temperature to achieve improved device properties.
One advantage of glass substrates is their low permeability to oxygen and water vapor, which reduces corrosion and other degradation of the OLED device. However, glass substrates are not suitable for certain applications in which flexibility is desired. In addition, manufacturing processes involving large glass substrates are typically slow and can therefore result in high manufacturing costs.
Recently, plastic substrates have been used in the fabrication of electronic devices. Plastic substrates have advantages over glass substrates because of their flexibility, light weight, thinness, and robustness. However, there can be certain technical challenges in fabricating electronic devices on plastic substrates. For example, the fabrication temperature typically must remain below the glass transition temperature, Tg, of the plastic substrate so that the substrate maintains its desirable physical properties, such as flexibility and transparency. In addition, plastic substrates typically have a relatively high coefficient of thermal expansion (CTE) compared to inorganic layers which may be applied in the fabrication process. A material's CTE indicates its expansion and contraction properties as a function of temperature. Furthermore, plastic substrates shrink after heating at elevated temperatures. Unlike thermal expansion, shrinkage is generally irreversible. Thermal expansion combined with shrinkage can therefore cause the article to curl significantly during heating and cooling processes, which may pose significant challenges during manufacturing.
Known electronic devices with plastic substrates typically have another disadvantage relating to oxygen and moisture diffusion. For example, plastic substrates are generally not impervious to oxygen and water vapor, and thus may not be suitable for the manufacture of certain devices such as OLEDs which may benefit from such properties. In order to improve the resistance of these substrates to oxygen and water vapor, coatings comprising ceramic materials have been applied to a surface of the plastic substrate. However, the interface between polymeric and ceramic layers is typically weak due to the incompatibility of the materials, and the layers are prone to be delaminated.
Accordingly, there is a need to provide flexible electronic devices that are robust against degradation due to environmental elements. There is also a need for reducing or preventing the stress and curl which may result from manufacturing processes employing thin film materials with varying CTEs.